In medical fields, emission computed tomography (ECT: Emission Computed Tomography) apparatus is used that detects radiation (such as gamma rays) emitted from radiopharmaceutical that is administered to a subject and is localized to a site of interest for obtaining sectional images of the site of interest in the subject showing radiopharmaceutical distributions. Typical ECT apparatus includes, for example, a PET (Positron Emission Tomography) device and an SPECT (Single Photon Emission Computed Tomography) device.
A PET device will be described by way of example. The PET device has a group of radiation detectors having block radiation detectors arranged in an arc shape. The group of radiation detectors is provided for surrounding a subject, and allows detection of radiation that is transmitted through the subject.
Among PET devices, a PET-Mammography device (hereinafter, referred to as a PET-Mammo device) for conducting a breast cancer physical examination has a characteristic that more doses of radiation fly from outside of a gantry toward the apparatus in comparison with typical PET devices. Accordingly, the PET-Mammo device has a shield that shields entering of radiation flying from outside of the gantry. Description will be given of a configuration of a conventional PET-Mammo device. As shown in FIG. 14, the conventional PET-Mammo device 50 has a gantry 51 with an opening for introducing a site of interest B of a subject M, a group of radiation detectors 52 that is provided so as to surround the opening of the gantry 51 on an outer periphery thereof and detects radiation, and a ring-shaped shield 53 on one end of the group of radiation detectors 52 that is adjacent to the subject M.
It should be noted that, in the PET-Mammo device 50, a whole body of the subject M is not introduced into the opening of the gantry 51. Radiopharmaceutical is administered to the subject M by injection in advance for conducting diagnosis with the PET-Mammo device 50. The radiopharmaceutical is to be distributed over the whole body of the subject M. Specifically, radiation is emitted from the whole body of the subject M, and flies toward the group of radiation detectors 52 from not only the site of interest B inside the gantry but also a site other than the breast of the subject M. Radiation 54 derived from outside of the gantry is obstructive to imaging of radiopharmaceutical distributions in the site of interest B. Accordingly, in order to obtain a sectional image more suitable for diagnosis, the conventional mammo-PET apparatus 50 has a ring-shaped shield 53 on one end of the group of radiation 52 adjacent to the subject M that prevents radiation 54 derived from outside of the gantry from entering into the group of radiation detectors 52 (see, for example, Patent Literature 1.)